The most common method for on-site treatment of domestic sewage and wastewater is a conventional septic system using a septic tank for anaerobic treatment and a tile bed, raised bed, or sand filter for aerobic biofiltration. These solid particle aerobic filters are readily constructed, and are passive, single-pass biofilters which require little maintenance. However, even in ideal conditions, nitrate and phosphorus are released to the groundwater because the treated water cannot be collected for further treatment. Loading rates of potent wastewater such as septic tank effluent in solid particle media are low, usually 15 cm/day (cm3 volume/cm2 area), and treatment beds therefore require large volumes of filter media. A tile bed requires 80400 m3 of unsaturated soil, and a sand filter requires about 2535 m3 of sand and gravel. Significantly higher loading rates are required for the biofilter to be transportable. The physical characteristics of natural filter media such as soil and sand are highly variable. The large volumes and natural variations preclude pre-manufacturing the biofilters to consistent specifications so that performance can be guaranteed at any site. Aerobic package plants that are manufactured off-site are generally highly mechanical units with high capital cost and high maintenance requirements.
There is a need for a low-maintenance single-pass aerobic biofilter with a filter medium that has predictable physical properties and therefore predictable treatment performance. It should withstand high loading rates so it can be pre-manufactured to consistent specifications in a small volume and transported to site. Burial of the system and removal of nitrate and other undesirable contaminants after aerobic treatment is also advantageous.
In certain countries, polluted water is used directly for human consumption and cooking, resulting in sickness and death from water-borne diseases. There is a need for a low-cost, low-technology, and transportable aerobic treatment system which removes substantial amounts of biological pathogens.
An at-grade peat system uses natural peat as the filter medium and removes nutrients such as nitrogen and phosphorus. It requires a very specific peat and the loading rate for septic tank effluent is only 4-5 cm/day, thereby precluding central manufacture and transport of the .about.50-m3 volume. The peat also requires special handling to avoid over-compaction. The system cannot be buried and it removes a significant area of the property (.about.200300 m2) from use.
U.S. Pat. No. 5,049,265 (Boyd et al.), granted in 1991, uses biologically active young sphagnum peat in containers which can be buried. The increased water-holding capacity enables treatment to occur at what are stated to be "very high loading rates". The peat is mixed with a non-specific amount of peat fibre to reduce the tendency to clog and pond on the surface. It is compacted by a non-specific amount to prevent channelling if undercompacted, and clogging if over-compacted. Because the medium is inconsistent treatment performance cannot be assured. Loading rates of only 715 cm/day are cited with a preferred rate of &lt;11 cm/day, which is insufficient to allow pre-construction and transport of the 2030 m3 volume to site.
Synthetic filter media have been used for treating relatively clear water. In U.S. Pat. No. 4,427,548 (Quick), granted in 1984, a slab of polyurethane foam is used as a physical and biological filter to remove solids and ammonium from aquarium water. The slab filter must be removed and cleaned frequently and does not constitute an alternative biofilter for treating potent wastewater with high solids and biochemical oxygen demand. Under high loading rates of potent wastewater, solid foam soon plugs up and becomes anaerobic, similar to a solid particle biofilter. German patent number DE-3,235,600 (REIMANN), which was published Mar. 29, 1984, shows foam being used in an aerobic reaction-promoting context.